When you go to a gym and you see people who load too much weight up on a machine and then proceed to do 1- 4 repetitions with extremely poor form and improper technique, which generally leads to hilarious outcomes.

Also used to describe people who load up machines to make themselves seem strong. but in reality are just weak and making a fool of themselves

Mate "Hey dude check out that guy over there trying to dumbbell curl 20 Kg's"
Me "Hahaha yeah, he's got more swing happening than a major league baseballer. What a one rep champion!"

Mate "look at Herp, he loaded up 150kg's on bench"
Me "...........yeah but if his spotter wasn't there he wouldn't be able to do half of that weight"
Mate " Look at him, whatta head!"
Me "ONE REP CHAMPION"